1. Field of the Invention
The present invention relates to a charge-up measuring apparatus used with an ion beam irradiation apparatus for irradiating a substrate with an ion beam for conducting treatment of ion implantation, etc., an ion implantation apparatus for irradiating a semiconductor substrate with an ion beam for forming MOS field effect transistors (MOSFET) on the surface of the semiconductor substrate, or the like for measuring charge-up of the substrate (charge phenomenon) with ion beam irradiation and more particularly to a charge-up measuring apparatus for measuring charge-up of the substrate simulatedly.
2. Description of the Related Art
Hitherto, by executing ion implantation into a semiconductor substrate, a semiconductor device, for example, a MOS field effect transistor 10 as shown in FIG. 8 has been formed on the surface of the semiconductor substrate.
To put it simply, the MOS field effect transistor 10 is manufactured by executing steps of forming a gate oxide film 4 and an element separation oxide film 5 on the surface of a semiconductor substrate (for example, silicon substrate) 2 according to a predetermined pattern, forming a gate electrode 6 on the surface of the gate oxide film 4, and using the gate electrode 6 as a mask to execute ion implantation of dopant (doped impurity) ions into both sides of the gate electrode 6, thereby forming a predetermined pattern of two impurity-doped layers 8. One impurity-doped layer 8 becomes a source and the other impurity-doped layer 8 becomes a drain.
The portion of the MOS field effect transistor 10, where the gate oxide film 4 is sandwiched between the gate electrode 6 and the semiconductor substrate 2, forms a capacitor structure and at the dopant ion implantation time, the positive charges transported accordingly are accumulated in the gate electrode 6 .
There is a tendency to make fine such a MOS field effect transistor 10; as the MOS field effect transistor 10 is made fine, the gate oxide film 4 becomes thin. For example, the gate oxide film 4 is made of silicon oxide and plays an important role in determining the MOS field effect transistor 10. As the gate oxide film 4 becomes thin, the withstand voltage of the gate oxide film 4 lessens. For example, the gate oxide film 4 is about 50 nm thick, in which case when charges are accumulated in the gate electrode 6 with ion implantation and the voltage becomes in the vicinity of 5 V, an electric current starts to flow penetrating the gate oxide film 4. The amount of charges penetrating the gate oxide film 4 are lessened as much as possible, leading to enhancement of the reliability of the gate oxide film 4 and prolong the life of the MOS field effect transistor 10.
To suppress positive charge-up (charge accumulation) caused by ions as described above, an ion implantation apparatus usually is provided with an electron supply source for supplying low energy electrons for neutralization to an ion beam in the upstream vicinity of the doped substrate. One of the electron supply sources is a plasma supply source for supplying plasma containing low energy electrons to an ion beam.
If such an electron supply source or a plasma supply source is provided, when insufficient electrodes are supplied, positive charge-up occurs on the substrate surface and when excessive electrons are supplied, negative charge-up occurs on the substrate surface. Thus, preferably the electron supply amount is controlled. To do this, at the first stage, it is necessary to measure the charge-up state on the substrate.
One of such charge-up measuring techniques is a technique wherein a charge-up measurement device is formed on the surface of a substrate and after ion implantation, the characteristics of the device are checked for measuring the charge-up state at a later time. However, in this technique, the doped substrate needs to be taken out from the vacuum vessel for measurement. Thus, checking the charge-up state requires at least several hours; this is a large problem.
To solve such a problem, JP-A-10-40856 proposes a charge-up measuring apparatus wherein a plurality of measurement conductors (beam collectors) placed in a state similar to the state of a substrate are connected to high resistance and the voltage of each measurement conductor at the ion beam irradiation time is measured, whereby the charge-up state of the substrate is measured simulatedly.
Generally, low energy electrons are used for ion beam neutralization. Specifically, the electrons emitted for neutralization from the electron supply source or the plasma supply source have an energy distribution, but most of the electrons are designed so as to become low energy electrons of about several eV, for example. The reason why they are designed so is that if successive electrons are supplied, the substrate surface is charged up negatively to the voltage corresponding to the electron energy and thus the charge-up voltage needs to be reduced. Therefore, it can be said that the orbit of the low energy electrons of about several eV in the vicinity of the substrate determines lessening charge-up of the substrate.
However, with the charge-up measuring apparatus described in JP-A-10-40856, there is a possibility that the voltage of the measurement conductor may become high as compared with the charge-up voltage of the substrate surface to be measured, because the charge-up voltage of MOS field effect transistor 10 of 5-V withstand voltage, for example, formed on the substrate surface does not become more than 5 V. If the voltage attempts to exceed 5 V, an electric current flows through the gate oxide film 4 and the voltage is lowered. However, since the measurement conductor is insulated and supported by an electrical porcelain, the voltage of the measurement conductor can become a high voltage of 5 V or more. The presence of such a high-voltage measurement conductor would have an effect on the orbit of the low energy electrons of about several eV described above.
Therefore, a difference occurs between the orbit of the electrons in the vicinity of the measurement conductor and the orbit of the electrons in the vicinity of the substrate surface to be measured, thus the voltage of the measurement conductor does not accurately reflect the charge-up state on the substrate surface. Therefore, the charge-up measurement precision of the substrate is degraded.
It is therefore an object of the present invention to provide a charge-up measuring apparatus capable of measuring charge-up of a substrate simulatedly and moreover with high accuracy.
According to a first aspect of the invention, there is provided a charge-up measuring apparatus comprising a plurality of measurement conductors being arranged on a plane crossing an ion beam for receiving the ion beam, a plurality of bidirectional constant-voltage elements being connected to the measurement conductors in a one-to-one correspondence, and a plurality of current measuring instruments each for measuring the polarity and magnitude of an electric current flowing through the corresponding bidirectional constant-voltage element.
According to a first aspect of the present invention, if the ion beam is not completely neutralized, each measurement conductor undergoes ion beam irradiation and is charged up positively or negatively and the voltage is raised positively or negatively. However, since the bidirectional constant-voltage elements are used, when the voltage of the measurement conductor is smaller than the breakdown voltage of the corresponding bidirectional constant-voltage element, the bidirectional constant-voltage element blocks the voltage of the measurement conductor and no current flows into the corresponding current measuring instrument. If any of the measurement conductors has charge-up voltage raised to the breakdown voltage, the bidirectional constant-voltage element connected to the measurement conductor is brought into conduction and the electric current of the polarity and the magnitude responsive to the charge-up of the measurement conductor flows through the bidirectional constant-voltage elements into the current measuring instrument corrected thereto. The current measuring instrument measures the polarity and the magnitude of the electric current. Therefore, each measurement conductor is placed in a state similar to that of the substrate, whereby charge-up of the substrate can be measured simulatedly.
Moreover, the charge-up voltage of each measurement conductor does not rise above the breakdown voltage of the corresponding bidirectional constant-voltage element as the bidirectional constant-voltage element conducts. That is, the charge-up voltage of each measurement conductor is limited to the breakdown voltage or less. Consequently, at the measuring time, the effect of the voltage of each measurement conductor on the orbit of low energy electrons can be lessened. Therefore, charge-up of the substrate can be measured simulatedly and moreover with high accuracy.
According to a second aspect of the present invention, a plurality of charge amount measuring instruments each for measuring the amounts of positive and negative charges flowing through the corresponding bidirectional constant-voltage element may be provided in place of the current measuring instruments. Thus, charge-up of the substrate can be measured simulatedly and moreover with high accuracy based on the passing-through charge amount.
According to a third aspect of the present invention, in order to use the charge-up measuring apparatus according to the invention with an ion implantation apparatus for forming a MOS field effect transistor on the surface of a semiconductor substrate by ion implantation, preferably the breakdown voltage of each bidirectional constant-voltage element is made almost equal to the withstand voltage of the gate oxide film of the MOS field effect transistor. Thus, the maximum charge-up voltage of each measurement conductor can be made almost equal to the maximum charge-up voltage of the MOS field effect transistor on the substrate surface, so that charge-up can be measured simulatedly and moreover with higher accuracy.
According to a fourth aspect of the present invention, a plurality of charge amount measuring instruments each for measuring the amounts of positive and negative charges flowing through the corresponding bidirectional constant-voltage element may be provided in place of the current measuring instruments. Thus, the amount of charges penetrating the gate oxide film of the MOS field effect transistor because of charge-up can be measured simulatedly with higher accuracy. Consequently, not only measurement of the charge-up state of the MOS field effect transistor 10, but also simulated measurement of a break mode based on the amount of charges passing through the gate oxide film can be made.